Many aircraft are required to have on-board flight recorders such as flight data recorders (FDRs), cockpit voice recorders (CVRs), and video recorders. Popularly known as the black box used for aircraft mishap or accident analysis, these units are also used to study air safety issues, material degradation, unsafe flying procedures, and jet engine performance.
In many airline accidents, or other aircraft accidents, the only systems that survive in a usable form are the crash-survivable memory units (CSMUs) of these flight recorders. The flight data recorder uses the CSMU to record specific aircraft performance parameters, such as airspeed, altitude, vertical acceleration, time, magnetic heading, control-column position, rudder-pedal position, control-wheel position, horizontal stabilizer, and fuel flow. In order to record these parameters, the physical conditions associated with each parameter must first be sensed. One of the sensors which are part of the flight recorder system is an accelerometer, or more particularly, a 3-axis accelerometer which can sense acceleration in three orthogonal axes: vertical axis (parallel with the gravity vector), lateral axis, and longitudinal axis.
An example of a known accelerometer used in flight recorder systems includes acceleration sensors which have pendulum weights suspended in fluids, with associated electronics and magnetics. These devices are fairly intricate in their mechanical detail, and are therefore relatively expensive to manufacture; and because they include fluids, they are not truly solid state and consequently have problems relative to reliability.
Solid state acceleration micro-electro-mechanical systems (MEMS) sensors are known which include the integration of mechanical elements and electronics on a common silicon substrate through microfabrication technology. While the electronics are fabricated typically using integrated circuit (IC) process sequences, e.g., CMOS, bipolar, or BICMOS processes, the micromechanical components are fabricated using compatible micromachining processes, or other processes such as electroplating, that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.
Such devices are described in U.S. Pat. No. 6,000,287 and typically include a sense element chip, interface electronics chip, blank substrate, ceramic chip carrier, and a cover enclosing the sense element chip, interface electronics, and substrate. These acceleration sensors use a capacitance bridge to sense capacitance change due to acceleration. In addition to being more reliable than the pendulum weights in fluid type acceleration sensor, such a capacitive approach allows several benefits when compared to the piezoresistive acceleration sensors. Such gaseous dielectric capacitors are relatively insensitive to temperature. The low thermal coefficient of expansion of many materials can produce a thermal coefficient of capacitance about two orders of magnitude less than the thermal coefficient of resistivity of doped silicon. Capacitance sensing therefore has the potential to provide a wider temperature range of operation, for a given error and without compensation, than piezoresistive sensing. Also, as compared with piezoelectric type accelerometers which require a dynamic input of some minimum frequency to generate a response, some capacitive sensing devices allow for response to DC accelerations, as well as dynamic vibration, which allows the capacitive accelerometer to be used potentially in a wider range of applications.
MEMS acceleration sensors have some of their own challenges. For example, there can be alignment errors between the sense element chip and blank substrate, between the blank substrate and ceramic chip carrier, between the ceramic chip carrier and circuit board on which it is mounted, and between the circuit board and board mount. For a 3-axis accelerometer, having an acceleration sensor sensing acceleration in each of three axes (vertical, lateral, longitudinal), these misalignments can affect the accuracy of a reading. For example, a strictly vertical acceleration should be indicated by the vertical acceleration sensor only; however, the misalignment of the accelerometer can cause the other acceleration sensors to sense a lateral acceleration, and a longitudinal acceleration, whose magnitudes depend on the amount and direction of the misalignment in the corresponding axes.
Other errors can occur in accelerometers, such as temperature effects and electronic offset. These errors are not restricted to MEMS acceleration sensors but can be found in some degree in other types of accelerometers such as piezo-film or piezoelectric sensor/acceleration transducers, suspended cantilever beam or proof mass, also known as seismic mass, shear mode accelerometer, thermal, bulk micromachined capacitive, bulk micromachined piezo resistive, capacitive spring mass based, electromechanical servo, null-balance, strain gauge—PCB piezotronics, resonance, magnetic induction, optical, surface acoustic wave (SAW), and laser accelerometers.
What is needed in the art is an improved accelerometer method and apparatus with error compensation.